I am using a transcend 64gig XC-1 u3 sd card with R95 and W60. I get the lockdown when viewing my 4k video. I have not tried formatting yet. Thanks for your heads up.

Since everything in my setup is standardised on HD720p, I can merely SUGGEST how to localise the problem.

Can you view the 4k video in the camera? If you can, then it is not the card. If you can't view 4k video in the camera - has the manual any recommendation?

On my Toshiba Satellite L650-D, I can preview HD720p smoothly, and edit and render it without lockups. I can preview HD1080p with bad stuttering, but I can edit and render it (more slowly), and then view the result without stuttering.

Streaming HD1080p (NASA at full steam) constantly pauses to catch up. Safest bit rate is something like 450 kbs for minimum interruptions. Trying to watch streaming 4k video crashes Internet Explorer, and I have to use the Task Manager to get out of IE.

This is not a new experience. My Canon M100 DV camcorder bought in 1998 has a firewire output, but not until 2008 could I get an AFFORDABLE laptop that could make use of it.